Primary and Secondary Skin Lesions
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Direct answer
Primary and secondary skin lesions are the standard morphological terms used to describe what a dermatologist sees on the skin. Primary lesions arise directly from the disease process, and their size, surface and depth define them, with a macule under 1 cm being the basic flat colour change. Secondary lesions develop later, through trauma, healing or chronic inflammation, and include scale, crust and lichenification.
What you must remember
- Macule: a flat, non-palpable change in colour, under 1 cm across; a patch is the same kind of lesion but over 1 cm.
- Papule: a solid, palpable elevation under 1 cm, such as a lichen planus papule.
- Plaque: an elevated, flat-topped lesion over 1 cm, often formed when papules coalesce.
- Nodule: a solid, firm lesion that extends deeper into the dermis or subcutis than a papule.
- Vesicle and bulla: fluid-filled elevations; a vesicle is under 0.5 cm and a bulla is larger than 0.5 cm (some texts, such as Fitzpatrick, use 1 cm).
- Pustule: an elevation containing visible pus, seen in impetigo and folliculitis.
- Erosion and ulcer: an erosion loses only epidermis and heals without scarring, whereas an ulcer extends into the dermis and heals with a scar.
- Secondary changes: scale, crust (dried exudate, honey-coloured in impetigo), excoriation and lichenification (thickened skin with accentuated markings from rubbing).
A size-based way to name a lesion
Most lesion questions turn on size, surface and depth, and the same features are recorded in every case note. Work through the descriptors in this order when you examine a patient.
| Lesion | Size cut-off | Defining feature |
|---|---|---|
| Macule | under 1 cm | flat, non-palpable colour change |
| Patch | over 1 cm | flat, non-palpable colour change |
| Papule | under 1 cm | solid and palpable |
| Plaque | over 1 cm | elevated and flat-topped |
| Nodule | variable | solid, extends deeper than a papule |
| Vesicle | under 0.5 cm | clear fluid-filled elevation |
| Bulla | over 0.5 cm | larger fluid-filled elevation |
Depth matters as much as size, because a nodule is defined by its extension into deeper tissue. Record whether the surface is smooth, scaly or crusted, since this changes the differential diagnosis.
Vignettes often describe a lesion in words and expect you to name it from size alone, so memorise the cut-offs exactly. A careful case note records size, shape, colour, surface, depth and distribution in that order, which makes the description reproducible for the next examiner.
Where lesion terms trip candidates
Lesion names are simple to learn but easy to misapply in clinical vignettes.
- Trap: calling a flat lesion of 1.5 cm a macule Correct: a macule is under 1 cm, so a flat lesion of this size is a patch.
- Trap: calling a shallow loss of skin an ulcer Correct: an erosion stops at the epidermis and heals without scarring, whereas an ulcer reaches the dermis.
- Trap: calling a 1 cm fluid-filled blister a vesicle Correct: a vesicle is under 0.5 cm, so a blister of this size is a bulla.
- Trap: listing lichenification as a primary lesion Correct: it is a secondary change from chronic rubbing and scratching, not a disease-defining primary lesion.
Frequently asked questions
What is the difference between a macule and a patch?
A macule and a patch are both flat, non-palpable colour changes, and the only difference is size. A macule measures under 1 cm, while a patch measures over 1 cm. Vitiligo and tinea versicolor produce macules and patches.
What size separates a vesicle from a bulla?
A vesicle measures under 0.5 cm across, whereas a bulla is larger than 0.5 cm. Both are fluid-filled elevations, so the size cut-off is the main distinguishing feature in bullous disease descriptions.
How does an erosion differ from an ulcer?
An erosion involves loss of the epidermis only and heals without scarring. An ulcer extends into the dermis and heals with a scar. Candidates should note this distinction when describing a healing or chronic lesion.
Which secondary lesion results from chronic rubbing and scratching?
Lichenification results from chronic rubbing and scratching, and it produces thickened skin with accentuated skin markings. It is typical of chronic eczema and lichen simplex chronicus. Excoriation, the linear scratch marks from the same process, is also a secondary change.
What is the difference between a papule and a plaque?
A papule is a solid elevation under 1 cm, whereas a plaque is an elevated, flat-topped lesion over 1 cm. Plaques often form when papules coalesce, as seen in psoriasis.